= Solution
The same base first makes $\mathrm{PhC^-(OPh)CN}$. This time addition occurs at a <carbon> bearing <hydrogen> para to the <nitro group>, giving a <sigma-H adduct>. The attacked <carbon> temporarily bears both its original H and the new <carbon> substituent. Negative charge is again delocalized through the ring toward the nitro oxygens.
A further base removes the <hydrogen> on that aromatic addition site. Its C–H <electrons> form an exocyclic bond toward the incoming <carbon>, while that <carbon> expels <phenoxide>. This is a base-induced elimination across the new ring–side-chain bond. Redistribution of the <nitronate>/quinoid pi system restores an <aromatic ring> and gives the side-chain carbanion $\mathrm{p{-}NO_2C_6H_4{-}C^-(Ph)CN}$. <protonation> during acidic workup yields \b[$\mathrm{p{-}NO_2C_6H_4{-}CH(Ph)CN}$].
This is <vicarious nucleophilic substitution>: the ring loses <hydrogen> while the <nucleophile> sacrifices its phenoxy leaving group. \b[<hydrogen> is not expelled as hydride], and the product's side-chain <hydrogen> can be supplied on workup rather than moving directly from the ring in a compulsory hydride shift. Base and isotope experiments in the primary mechanistic work support reversible addition followed by base-induced elimination, while not uniquely choosing every E2/E1cb limit.<Reaction of organic anions. Part 108. On the mechanism of the vicarious nucleophilic substitution of hydrogen in nitroarenes by Mieczysław Mąkosza and Tomasz Glinka>[1983, pp. 3860–3861, isotope effects, base dependence and hydrogen/halogen competition]{ref} The displayed para product is consistent with access to the less crowded activated position.
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